Everyone knows the sex stereotypes: boys like to play sports; girls like to play house. These beliefs are so entrenched that even parents have bought in: new research shows that preschool girls are 16% less likely than boys their age to be taken outside by their parents to play.

Nearly half of preschoolers don’t venture outside with their parents to play every day, and girls are most likely to suffer, according to a new study of 8,950 U.S. kids in their final year before entering grade school.

Is the discrepancy a result of societal norms? Do boys demand to be taken outside more? Are girls just assumed to be less athletic and less inclined to run around outdoors?

It’s likely a combination of all three, says lead author Pooja Tandon, a pediatrician and researcher at Seattle Children’s Hospital, which is affiliated with the University of Washington. “Children need outdoor time every day, and they need more outdoor play opportunities than they’re getting,” says Tandon.

According to the study, which was published in the Archives of Pediatric & Adolescent Medicine:

Previous research has shown there is a sex disparity in physical activity levels, with boys being more active than girls from a young age and substantial declines in girls’ physical activity as they get older.

One philosopher argued that “gendered standards of cleanliness” and play leave girls less exposed to microorganisms commonly found in outdoor environments and may be an explanation for the higher rates of atopic and autoimmune diseases in females.

Given the association between outdoor time and physical activity and numerous potential other benefits, our findings support giving particular attention to the study and promotion of outdoor play for girls.

The research contains other disturbing findings. Minorities were far less likely to spend time on the playground with their parents than kids with white parents: Asian mothers were 49% less likely, black mothers 41% less likely and Hispanic mothers 20% less likely to venture outside with their kids. It’s probably not coincidence that minority children have a greater tendency to be overweight.

Children who are cared for at home also appear to be bigger couch potatoes. Although preschoolers should get at least 60 minutes of physical activity every day, 42% of those who don’t regularly attend child care don’t go outside daily. Eighty percent of the kids in the study attended some sort of child care, averaging 28 hours a week there.

But Tandon cautions that not all preschools engage in enough outdoor play. “There are high-quality child care centers where kids go out regardless of weather, but in a lot of settings, they do more indoor recesses,” says Tandon. “Research suggests that kids are sedentary 80% of the time when they are in preschool.”

The situation only worsens once they enter elementary school. In December, a study found that only six states — Alabama, Georgia, Mississippi, North Carolina, Illinois and Iowa — adhere to standards from the National Association of Sports and Physical Education that schoolchildren participate in 150 minutes a week of physical education. And just three states — Delaware, Virginia and Nebraska — have 20 minutes of mandatory elementary-school recess a day.

Being active inside is great but it’s not a substitute for the great outdoors. “If you get outdoors, you’re more likely to be active,” says Tandon. Other research suggests that being outdoors is important for motor development, while contact with nature is beneficial for mental health and cognitive development. “One thing that is lost in all this is the opportunity for free play,” says Tandon. “For young children, exercise and play is interrelated. Being outdoors is more conducive to both.”

Moms were more likely than dads to take their kids outside, and gym-rat moms even more so: mothers who worked out more than four times per week were 50% more likely to take their child outside at least once a day than mothers who didn’t exercise.

The lack of outside playtime is a reflection of how childhood has evolved. Children’s lives, in general, are far more structured than they were a generation ago. Kids have after-school classes — some even attend before-school enrichment — and they’re tethered to the house, in contrast to years past when they’d wander the neighborhood with friends after school. And with more two-parent working families than in the past, an increasing number of parents are feeling squeezed for time. If the choice is between making dinner and taking your kids to the park, it’s likely that meal prep will win out. After all, kids have to eat. They don’t have to go outside. But they should.